Quantifying resistant starch using novel, in vivo methodology and the energetic utilization of fermented starch in pigs.

To quantify the energy value of fermentable starch, 10 groups of 14 pigs were assigned to one of two dietary treatments comprising diets containing 45% of either pregelatinized (P) or retrograded (R) corn starch. In both diets, a contrast in natural ¹³C enrichment between the starch and nonstarch components of the diet was created to partition between enzymatic digestion and fermentation of the corn starch. Energy and protein retention were measured using indirect calorimetry after adapting the pigs to the diets for 3 wk. Fecal ¹³C enrichment was higher in the R-fed pigs (P < 0.001) and 43% of the R resisted enzymatic digestion. Energy retained as protein was unaffected and energy retained as fat was 29% lower than in P-fed pigs (P < 0.01). Prior to the morning meal, end products of fermentation substantially contributed to substrate oxidation in the R-fed pigs. During the 3-4 h following both meals, heat production was higher (P < 0.05) in P-fed pigs, but this was not preferentially fueled by glucose from corn starch. Digestible energy intake, metabolizable energy intake, and energy retention were reduced (P < 0.05) in R-fed pigs compared with P-fed pigs by 92, 54, and 33 kJ/(kg⁰·⁷⁵ · d), respectively. Therefore, the energy values of fermented resistant starch were 53, 73, and 83% of the digestible, metabolizable, and net energy values of enzymatically degradable starch, respectively. Creating a contrast in natural ¹³C enrichment between starch and nonstarch dietary components provides a promising, noninvasive, in vivo method for estimating the proportion of dietary starch fermented in the gastrointestinal tract.